Support arm for a surgical theater system

ABSTRACT

A surgical theater system includes a boom rotating about a hub and coupled to a counterbalance arm assembly having a counterbalance arm and a monitor mount assembly configured to permit a monitor or other device to be positioned horizontally and vertically within a surgical suite and maintain the vertical elevation selected. The counterbalance arm includes a proximal hub, a first parallel link, a second parallel link, a spring link, a spring and a distal hub. First, second and third proximal pivot axes extend through the proximal hub with the third pivot axis being movable along a radial line forming an angle with a line intersecting the first and second proximal pivot axes. The spring link is coupled to the third proximal pivot axis, spring and second parallel link.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of international applicationserial no. PCT/US01/28731 filed Sep. 14, 2001.

BACKGROUND AND SUMMARY

This invention relates generally to support arms for surgical theatersystems and more particularly to support arms having a counterbalancearm to balance the weight of the supported object so that it can besuspended at a selected height without locking the arm.

Surgical theater systems are floor, wall, or ceiling mounted systemsincluding devices which can be positioned in desired locations within asurgical suite to maximize there accessibility, utility, and/orviewability. Examples of surgical light systems can be found in U.S.Pat. Nos. 6,012,821 and 6,132,062 and U.S. applications Ser. Nos.09/050,265; 09/050,529; 09/050,534 and 09/747,605 which are expresslyincorporated herein by reference. Such surgical theater systems supportdevices such as surgical lights, monitors, cameras, and electronicinstruments utilized by surgeons and operating room personnel during theperformance of surgical procedures. The above identified patentsdemonstrate that it is well known to provide surgical theater systemswhich facilitate the positioning, both horizontally and vertically, ofthe devices attached thereto to maximize their utility. In order tomaintain the proper orientation in space of the device mounted to thecounterbalance arm, the counterbalance arm includes a parallelogramlinkage.

U.S. application Ser. No. 09/747,605 shows a surgical theater systemwith a counterbalance arm which seeks to minimize vertical movement of adevice after it has been positioned at a desired height by providing acounterbalance arm having frictional brakes engaging pivot pins aboutwhich the arm rotates. Counterbalance arms are well known devicesincorporating counterweights or spring mechanisms which provide avertical upward force to the distal end of a cantilevered arm tocounteract the downward force exerted on the distal end of thecantilevered arm by the object supported. The forces create torquesabout the lower pivot axis of the counterbalance arm. Ideally, thetorque exerted on the counterbalance arm will be equal in magnitude to,but Opposite in direction to, the torque exerted on the counterbalancearm by the weight. Recognizing that the ideal situation cannot always beachieved, U.S. application Ser. No. 09/747,605 compensates for theinequalities by applying a frictional force to the pivot axis whichpermits the torques of the weight and the spring members to be slightlyunbalanced.

The torque exerted by a weight hanging freely suspended from a fixedpoint on an arm is proportional to the sine of the angle formed betweenthe vertical plane and the longitudinal axis of the arm. Thus thegreatest torque is exerted by an object freely hanging from a fixedpoint on a beam when the longitudinal axis of the beam is horizontal,sin 90 deg=1.

Springs and gas springs typically obey Hooke's law, at least while inthe region of elasticity, so that the force exerted by the spring isproportional to its displacement from equilibrium. In counterbalancearms, the spring is typically not mounted perpendicular to thelongitudinal axis of the arm but rather couples to the distal end of thearm and to an end of a spring link. The spring link is typicallypivotally mounted at a first end to a fixed location relative to thepivot axis of the proximal end of the counterbalance arm and mounted atthe other end to slide within a slot formed in the counterbalance arm.The spring is typically pivotally mounted to the sliding end of thespring link and pivotally mounted to the second end of thecounterbalance arm. Thus the spring exerts a force directedsubstantially along the longitudinal axis which is converted by thespring link into components of force parallel to and perpendicular tothe longitudinal axis of the spring link. The component parallel to thelongitudinal axis creates strain along the link which is assumed to beincompressible. The component perpendicular to the longitudinal axis ofthe spring link induces the pivot pin sliding in the slot intoengagement with the upper edge of the slot so that the perpendicularcomponent reduces to a component perpendicular to the longitudinal axisof the counterbalance arm and a component parallel to the longitudinalaxis of the counterbalance arm. In the disclosed embodiment of acounterbalance arm, second parallel link is formed so that a pin extendsthrough the longitudinal slot and a roller which engages an upper wallof the second parallel link to transfer this force to the secondparallel link through the roller rather than through engagement of thepin with the upper wall of the slot. The component perpendicular to thelongitudinal axis of the counterbalance arm creates a torque about thepivot axis at the proximal end of the counterbalance arm which ispreferably substantially equal in magnitude to, but opposite indirection to, the torque induced by the weight.

Unfortunately, gas springs do not always obey Hooke's law preciselythroughout the full stroke range, so it is difficult to design acounterbalance arm that cancels out the torque induced by the weight ofthe object suspended by the arm. Hooke's Law provides a linear equationrelating the force exerted by an elastic medium to the displacement fromequilibrium of the medium. In: gas springs the displacement can berepresented by the stroke length of the piston. Hooke's Law states thatforce (F) of a spring is equal to the displacement of the spring fromits equilibrium position (x) times a spring constant (k) or:F=kx.However, it has been found that gas springs are typically manufacturedso that near the ends of their stroke lengths, the gas spring is nolonger operating in the linear region of the displacement v. forcecurve.

In accordance with one embodiment of the disclosed surgical theatersystem, a counterbalance arm is provided which includes an adjustmentpin set at an angle to the vertical for modifying the offset of theproximal end of a spring link from the proximal end of a first parallellink so that the proximal end of the spring link is not mounted at afixed location relative to the pivot axis at the proximal end of thecounterbalance arm. The angle that adjustment pin forms with thevertical introduces a mechanical disadvantage to the springlink/spring/slot combination to compensate for increased spring forcesgenerated when the counterbalance arm is lowered from the raised to alowered position.

In accordance with one embodiment, the counterbalance arm includes afirst parallel link, a second parallel link, a spring link and a spring.The first parallel link is mounted at the proximal end to pivot about aproximal pivot axis and mounted at a distal end to pivot around a distalpivot axis. The second parallel link is mounted at its proximate end forpivotal movement about a second proximate pivot axis and at its distalend for pivotal movement about a second distal pivot axis. Thedisplacement between the first and second proximal pivot axes and thedisplacement between the first and second distal pivot axes issubstantially equal to maintain the parallel relationship between thefirst and second parallel links. The spring link is mounted at itsproximal end to pivot about a third proximal axis and at its distal endto slide relative to the second parallel link. The third proximal axisextends through a pivot pin which is movable relative to the firstproximal pivot axis to adjust the displacement between the third andfirst proximal pivot axes.

Additional features and advantages of the support arm for a surgicaltheater will become apparent to those skilled in the art uponconsideration of the following detailed descriptions exemplifying thebest mode of carrying out the apparatus as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing the disclosed invention reference will be made to thefollowing drawings in which:

FIG. 1 is a perspective view of a surgical theater system having amonitor mounted by a monitor mount to a counterbalance arm;

FIG. 2 is a front plan view of the counterbalance arm and monitor mountof FIG. 1;

FIG. 3 is a sectional view along line 3—3 of the counterbalance arm andmonitor mount of FIG. 2;

FIG. 4 is a detailed view of a portion of the counterbalance arm of FIG.3;

FIG. 5 is an exploded view of the counterbalance arm of FIG. 1;

FIG. 6 is a cross-sectional view of the counterbalance arm in its raisedposition showing the adjustment screw rotated completely in a firstdirection to place the third proximal pivot axis extending through theproximal end of the spring links in its uppermost position;

FIG. 7 is a cross-sectional view of the counterbalance arm in its raisedposition showing the adjustment screw rotated completely in a seconddirection to place the third proximal pivot axis extending through theproximal end of the spring links in its lowermost position;

FIG. 8 is a cross-sectional view of the counterbalance arm of FIG. 7showing the arm pivoted downwardly about the first proximal pivot axisso that the arm is in a substantially horizontal position; and

FIG. 9 is a cross-sectional view of the counterbalance arm of FIG. 7showing the arm pivoted downwardly about the first proximal pivot axisso that the arm is in its lowered position.

DETAILED DESCRIPTION OF THE DRAWINGS

A surgical theater system 10 in accordance with one embodiment of thedisclosed device is provided for mounting a device 12, such as a monitor14, within a surgical suite so that the device can be moved horizontallyand vertically within the suite to facilitate maximum utilization ofdevice 12. For example, when a monitor 14 is to be used by a surgeonduring a surgical procedure, surgical theater system 10 permits monitor14 to be positioned for optimal viewing by the surgeon withoutinterfering with the surgical procedure. While the embodiment of thesurgical theater system disclosed herein mounts a monitor 14 within asurgical suite, it is within the teaching of the disclosure for otherdevices 12, such as surgical lights, cameras, etc., to be mounted tosurgical theater system 10 and for surgical theater system to be locatedin other rooms of a facility in which the device 12 is to be used.

Illustratively, surgical theater system 10 is a ceiling mount systemhaving a central hub 16 mounted for rotation about a rotational axis 18extending through the hub 16 and a structural support (not shown) in theceiling 20. A boom 22 is mounted to hub 16 and includes a proximal end23, a distal end 24, a horizontally-extending portion 26, and avertically-extending portion 28. Boom 22 is mounted at proximal end 23to hub 16. Horizontally-extending portion 26 extends radially from hub16. Vertically-extending portion 28 extends downwardly fromhorizontally-extending portion 26 so that distal end 24 of boom 22 ishorizontally and vertically displaced from hub 16. A counterbalance armassembly 30 is mounted at its proximal end 32 to distal end 24 of boom22 for rotation about an axis 34 extending longitudinally throughvertically-extending portion 28 of boom 22.

Counterbalance arm assembly 30 includes a bearing housing 36, a proximalhub 38, a counterbalance arm 40, a distal hub or monitor pivot 48, and amonitor mount assembly 50. Counterbalance arm 40 pivots about a proximalpivot axis 54 and a distal pivot axis 56 so that a device 12 attached tomonitor mount assembly 50 can be raised and lowered with respect to theboom 22. Monitor mount assembly 50 is mounted to monitor pivot 48 forrotation about rotation on axis 58 permitting monitor 14 to bepositioned at a desirable location within a suite.

Referring to FIGS. 2 and 3, counterbalance arm 40 includes an armhousing 42 which acts in the illustrated embodiment as a first parallellink, a counterbalance arm weldment 44 which acts in the illustratedembodiment as a second parallel link, a plurality of spring links 46,and a gas spring 52. Arm housing or first parallel link 42 is mounted atits proximal end 60 to proximal hub 38 for pivotal movement relative toproximal hub 38 about a first proximal pivot axis 54. Arm housing orfirst parallel link 42 is mounted at its distal end 62 to distal hub ormonitor pivot 48 for pivotal movement relative to monitor pivot 48 aboutfirst distal pivot axis 56. Arm weldment or second parallel link 44 ismounted at its proximal end 64 to proximal hub 38 for pivotal movementrelative to proximal hub 38 about a second proximal pivot axis 66. Armweldment or second parallel link 44 is mounted at its distal end 68 todistal hub or monitor pivot 48 for pivotal movement relative to monitorpivot 48 about a second distal pivot axis 70. A displacement 72 betweenfirst and second proximal pivot axes 54, 66 is fixed as is adisplacement 74 between first and second distal pivot axes 56, 70.Because the displacements 72, 74 of the respective pivot axes 54, 66,56, 70 are fixed, a longitudinal axis 76 of the first parallel link 42and a longitudinal axis 78 of the second parallel link 44 are maintainedsubstantially parallel to each other during pivotal movement ofcounterbalance arm 40.

Illustratively, arm weldment or second parallel link 44 is formed from arectangular metal tube having a top wall 80, a bottom wall 82, and twoside walls 84, 86 as shown in FIG. 5. Four ears 88, 90 are coupled toside walls 84, 86. Illustratively, two proximal ears 88 are attached tothe side walls 84, 86 of, and extend longitudinally beyond, the tube atthe proximal end 64 of arm weldment 44. Similarly, two distal ears 90are attached to the side walls 84, 86 of, and extend longitudinallybeyond, the tube at the distal end 68 of arm weldment 44. Each proximalear 88 is formed to include a pivot pin-receiving hole 92. Each distalear 90 is formed to include a pivot pin-receiving hole 94 and anattachment pin-receiving hole 96. A second proximal pivot pin 98 extendsthrough pivot pin-receiving hole 92 to couple proximal end 64 of armweldment 44 to proximal hub 38 for pivotal movement relative to proximalhub 38 about second proximal pivot axis 66. A second distal pivot pin100 extends through pivot pin-receiving hole 94 to couple distal end 68of arm weldment 44 to distal hub 48 for pivotal movement relative todistal hub 48 about second distal pivot axis 70. An attachment pin 102extends through each attachment pin-receiving hole 96 and a distal endclevis 104 of gas spring 52 to pivotally couple gas spring 52 to armweldment 44. Each side wall 84, 86 of second parallel link 44 is formedto include a longitudinally-extending slot 106 having an upper wall 108,a lower wall 110, a distal end wall 112 and a proximal end wall 114.Upper, lower, distal end and proximal end walls 108, 110, 112, 114retain a connection pin 116 which is received for sliding movementbetween the proximal end wall 112 and distal end wall 114. Connectionpin 116 extends through slot 106, connection pin-receiving hole 124 inspring links 46, clevis 130 of gas spring 52, and a plurality of rollers117. Rollers 117 are sized to be received in the interior of armweldment 44 to facilitate movement of connection pin within slot and totransfer forces exerted on connection pin through the body of roller toupper wall of arm weldment 44. While described herein as being urgedagainst top wall 108 of slot 106 to transfer perpendicular forces to armweldment 44, in the illustrated embodiment connection pin 116 does notengage top wall 108 of slot 106, rather connection pin 116 transfersforces to arm weldment 44 through rollers 117. While illustrated as anarm weldment 44, it is within the teaching of the disclosure for secondparallel link 44 to be formed from a single link or a plurality oflinks.

Illustratively, each spring link 46 includes a proximal end 118 formedto include a pivot pin-receiving aperture 120 and a distal end 122formed to include a connection pin-receiving aperture 124. A third pivotpin 126, through which a third proximal pivot axis 128 extends, ispivotally coupled to proximal end 118 of spring link 46 by shoulderbolts 196 received in each of the pivot pin-receiving apertures 120 inthe proximal end of each spring link 46 and two threaded holes 197 inopposite ends of third proximal pivot pin 126. Connection pin 116extends through each connection pin-receiving aperture 124 and throughslot 106 in arm weldment 44 to slidingly couple distal end 122 of eachspring link 46 to arm weldment 44. Connection pin 116 also extendsthrough proximal end clevis 130 of gas spring 52 to pivotally couple theproximal end of gas spring 52 to spring links 46 and slidingly couplethe proximal end of gas spring 52 to arm weldment 44. While theillustrated embodiment includes two spring links 46, it is within theteaching of the disclosure for a single spring link, a spring linkweldment or assembly, or a plurality of spring links to be provided.

In the illustrated embodiment, a single gas spring 52 is provided. Gassprings are well known in the art and are commercially available fromseveral sources. Illustratively, gas spring 56 is a series 16 gas springavailable from Suspa Incorporated. Gas spring 52 includes a cylinderhousing 132 having a clevis 130 extending from the proximal end and arod 134 extending into the cylinder housing 132 and having clevis 104 atthe distal end. Connection pin 116 extends through connectionpin-receiving apertures 124 in the distal end 122 of spring links 46,slots 106 in arm weldment 44 and the clevis 130 at the proximal end ofthe cylinder housing 132 of the gas spring 52 to pivotally couple thegas spring 52 to the distal end 122 of the spring links 42 and slidinglycouple the gas spring 52 to arm weldment 44. Connection pin 116 extendsthrough connection pin-receiving holes 124 in the distal ears 90 of armweldment 44 and through clevis 104 at the distal end of the rod 134 ofgas spring 52 to pivotally couple the distal end of gas spring 52 to armweldment 44 at a location between distal end wall 112 of slot 106 anddistal end 68 of arm weldment 44. While the terms proximal and distalend are used in conjunction with the gas spring 52, those skilled in theart will recognize that the orientation of the gas spring 52 is notcritical to the invention. The terms proximal and distal are used inthis written description to indicate relative proximity, following thevarious arms of the surgical suite apparatus, to the mounting location.In the claims, the terms proximal and distal distinguish betweenportions of various components (similar to the terms “first” and“second”) and are not intended to indicate proximity to any othercomponent of a surgical theater apparatus. Also, those skilled in theart will recognize that mechanical springs, elastic material, and othermaterials and devices for exerting a force may be substituted for gasspring within the teaching of this disclosure. All such devices aregenerally referred to herein as springs 136.

The proximal hub 38 is formed to include a shaft 138 and a hub portion140. Illustratively, shaft 138 is a tube coupled at its distal end tothe hub portion 140. Shaft 138 has a length 142, best seen in FIG. 3,sufficient to extend substantially through bearing housing 36, anoutside diameter 144 sized to receive two tapered roller bearings 146and a Rulon™ bearing 148, and an inside diameter 150 sized to receive apower and signal cable 152 of a slip ring assembly 154. Illustratively,a proximal end 156 of shaft 138 is threaded to receive a locknut withkey 158. Bearing housing 36 is formed to include an aperture 160extending longitudinally therethrough. Aperture 160 includes a centralbore 162 and two counterbores 164 at opposite ends of the bearinghousing 36. Counterbores 164 have an inside diameter sized to receivetwo bearing races 166 on which the rollers of the tapered rollerbearings 146 ride and a brake ring 168 against which Rulon™ bearing 148is pulled to adjust the ease of rotation of counterbalance arm assembly30 about rotation axis 34. Rulon™ bearing 148, brake ring 168, races 166and tapered roller bearings 146 are secured within bearing housing 36 bya lock washer with key 170 and locknut with key 158. Cable 152 of theslip ring subassembly extends through shaft 138 and through an aperture172 in hub portion 140 so that power and electrical signals can beconveyed to monitor 14.

Illustratively, hub portion 140 includes a disk portion 174 and aprotrusion 176 to which shaft 138 is attached. Disk portion 174 isformed to include a centrally located first pivot pin-receiving hole178, a second pivot pin-receiving hole 180 offset radially from firstpivot pin-receiving hole 178, a nut-receiving aperture 182, a pivotpin-receiving aperture 184, an adjustment screw head-receivingcounterbore 186, and an adjustment screw shaft-receiving bore 188. Thefoci of first and second pivot pin-receiving holes 178, 180 are locatedon a diametrical line 190 substantially parallel to rotation axis 34.Adjustment screw shaft-receiving bore 188 extends from a peripheral edge192 of disk portion 174 radially inward through adjustment screwhead-receiving counterbore 186, pivot pin-receiving aperture 184 andnut-receiving aperture 182, all of which are coaxial and concentricabout a radial line 194. Third pivot pin 126 includes a longitudinalaxis serving as third proximal pivot axis 128 and has an outsidediameter sized to be received in pivot pin receiving aperture 184.Threaded holes 197 extend longitudinally inwardly from opposite ends ofthird pivot pin 126 to receive two shoulder bolts 196 which extendthrough pivot pin-receiving apertures 120 of spring links 46 topivotally couple proximal ends 118 of spring links 46 to proximal hub 38for movement about third proximal pivot axis 128. Third pivot pin 126includes a threaded hole 198 extending radially therethrough. Thirdpivot pin 126 is received in pivot pin-receiving aperture 120 so thatlongitudinal axis 128 extends perpendicularly through disk portion 174and threaded hole 198 is aligned with adjustment screw shaft-receivingbore 188.

Referring now again to FIG. 3, distal hub 48 includes a disk portion 200and a mounting protrusion 202. Disk portion 200 is formed to include afirst distal pivot pin-recieving hole 204 and a second distal pivotpin-receiving hole 206. Illustratively, first distal pivot pin-receivinghole 204 is concentrically located with respect to disk portion 200.Second distal pivot pin-receiving hole 206 is radially displaced fromfirst distal pivot pin-receiving hole 204 by a displacement 208. Thefoci of first and second distal pivot pin-receiving holes 204, 206 arelocated on a diametrical line 210 substantially parallel to rotationaxis 58. Mounting protrusion 202 is formed to include an aperture 212having an inside diameter sized to receive a portion of a slip ring 214and cable 156 of slip ring subassembly. Aperture 212 is concentric aboutrotation axis 58.

Arm housing 42 includes a tubular central section 214 with a pair oflaterally spaced apart proximal ears 216 and a pair of laterally spacedapart distal ears 218 extending longitudinally beyond central section214 as shown in FIG. 5. Arm housing 42 is shown as a monolithicstructure but it is within the teaching of the disclosure for armhousing 42 to be formed from multiple components. Each proximal ear 216includes an outer rim 220 coupled by a plurality of spokes 222 to acentral hub 224 formed to include a first proximal pivot pin-recievinghole 226 extending therethrough. Illustratively, each distal ear 218 iscup-shaped with a central first distal pivot pin-receiving hole 228extending therethrough. A first proximal pivot pin 230 extends througheach first proximal pin-receiving hole 226 of arm housing 42 and thefirst proximal pivot pin-receiving hole 180 of proximal hub 38 to couplearm housing 42 to proximal hub 38 for pivotal movement about firstproximal pivot axis 54. A first distal pivot pin 232 extends througheach first distal pin-receiving hole 228 of arm housing 42 and the firstdistal pivot pin-receiving hole 204 of distal hub 48 to couple armhousing 42 to distal hub 48 for pivotal movement about first distalpivot axis 56. First proximal pivot pin 230 and first distal pivot pin232 intersect longitudinal axis 76 of arm housing 42 so that arm housing42 acts as a parallel link or parallel link assembly extending betweenfirst proximal pivot axis 54 and first distal pivot axis 56.

Illustratively, tubular central section 214 is sized to receive armweldment 44, spring links 46 and gas spring 52 and their associatedconnecting pins and mounting hardware therein. In the illustratedembodiment arm housing 42 protects arm weldment 44, spring links 46 andgas spring 52 and their associated connecting pins and mounting hardwarefrom the environment and permits counterbalance arm 40 to be easilycleaned. To facilitate this protective function, counterbalance arm 40comprises two caps 234 and two plugs 236 to further enclose arm weldment44, spring links 46 and gas spring 52 and their associated connectingpins and mounting hardware and the various pivot and connection pinscoupling these various components to the proximal and distal hubs 38 and48, respectively.

Monitor mount assembly 50 includes an upper tube monitor cradle weldment238, a lower monitor tube/handle weldment 240 and a handle mandrel 242.Upper tube monitor cradle weldment 238 includes a proximal rotation hub244, a horizontally extending arm portion 246, a downwardly extendingarm portion 248 and a distal rotation hub 250. Lower monitor tube/handleweldment 240 includes a rotation hub 252, an L-shaped bracket arm 254,an internally threaded handle receiving hub 256, and a monitor bracket258. Handle mandrel 242 is formed to include a longitudinally extendingaperture 260 communicating with a switch-receiving hole 262 in a distalend 264. Handle mandrel 242 is externally threaded at a proximal end 266to facilitate connection with handle receiving hub 256 of lower monitortube/handle weldment 240.

Mounting protrusion 202 of distal hub 48 is mounted to proximal rotationhub 244 of monitor mount assembly 50 for rotational movement of monitormount assembly 50 about rotation axis 58. To facilitate this rotationalmovement, thrust needle bearing 268 is sandwiched between an upperthrust washer 270 received in a cavity in the distal hub 48 ofcounterbalance arm 40 and a lower thrust washer 272 received in a cavityin the proximal rotation hub 244 of the monitor mount assembly 50. Abearing cover 276 is provided between distal hub 48 and proximalrotation hub 244 to protect thrust needle bearings 268. A spindle 278extends through proximal rotation hub 244, a steel washer 280, a flangedbushing 282, thrust needle bearings 268, thrust washers 270, 272 andinto the aperture 212 of distal hub 48 where it is secured by a setscrew 284 to distal hub 48. A lower cap 285 is provided to cover lowerend of proximal rotation hub 244. As shown, for example, in FIG. 3, asecond slip ring 286 of slip ring assembly 154 is received in anaperture 288 of proximal rotation hub 244 to facilitate rotation ofmonitor mount assembly 50 about rotation axis 58 and transfer of signalsand power through slip ring assembly 154 to monitor 14.

As shown, for example, in FIG. 3, horizontally-extending arm portion 246and downwardly-extending arm portion 248 are tubes coupled together inan L-shape. Aperture 288 of proximal rotation hub 244 communicates witha lumen 290 of horizontally-extending arm portion 246 anddownwardly-extending arm portion 248 to permit a second signal and powercable 292 of slip ring assembly 154 to extend from proximal rotation hub244 to distal rotation hub 250. The lumen 290 of downwardly-extendingarm portion 248 communicates with an aperture 294 extending horizontallythrough distal rotation hub 250 to permit second signal and power cable292 of slip ring assembly 154 to extend into distal rotation hub 250.

Lower monitor tube/handle weldment 240 is mounted at rotation hub 252 todistal rotation hub 250 of upper tube monitor cradle weldment 238 topermit rotational movement of lower monitor tube/handle weldment 240with respect to upper tube monitor cradle weldment 238 about a rotationaxis 296. While mounted for rotational movement about rotation axis 296,the presence of stops (not shown) limits movement of lower monitortube/handle weldment 240 with respect to rotation axis 296 to pivotalmovement having 145 degrees of travel. Stops may be positioned toprovide a greater or lesser range of pivotal motion within the teachingsof the disclosure. To facilitate pivotal movement of lower monitortube/handle weldment 240 about rotation axis 296, a thrust needlebearing 298 is sandwiched between a proximal thrust washer 300 receivedin a cavity in distal rotation hub 250 of upper tube monitor cradleweldment 238 and a distal thrust washer 302 received in a cavity inrotation hub 252 of lower monitor tube/handle weldment 240. A bearingcover 304 is provided between distal rotation hub 250 of upper tubemonitor cradle weldment 238 and rotation hub 252 of lower monitortube/handle weldment 240 to protect the thrust needle bearings 298. Aspindle 305 extends through rotation hub 252 of lower monitortube/handle weldment 240, a steel washer 306, a flanged bushing 308,thrust needle bearings 298, thrust washers 300, 302 and into aperture294 of distal hub 250 of upper tube monitor cradle weldment 238 where itis secured by a set screw 310 to distal hub 250. Two end caps 312 areprovided to enclose the outer ends of rotation hub 252 of lower monitortube/handle weldment 240 and distal rotation hub 250 of upper tubemonitor cradle weldment 238, respectively.

L-shaped bracket arm 254 of lower monitor tube/handle weldment 240 isillustratively comprised of tubes coupled together in an L-shape. Anaperture 314 of rotation hub 252 of lower monitor tube/handle weldment240 communicates with the a lumen 316 of L-shaped bracket arm 254 topermit second signal and power cable 292 of slip ring assembly 154 toextend from rotation hub 252 to a distal end 318 of L-shaped bracket arm254. Signal and power cable 292 extends from distal end 318 of L-shapedbracket arm 254 and is electrically coupled to a 23-pin connector block320. A headed pin 322 is coupled to monitor bracket 258 to facilitateattachment of a monitor cable assembly 324 to monitor bracket 258.Monitor cable assembly 324 mates with 23-pin connector block 320 50 thatpower and video signals can be provided to monitor 14. Illustratively,monitor cable assembly 324 includes a plurality of cables terminating inconnectors adapted for attachment to inputs of a monitor 14. A controlwire 326 is coupled to monitor cable assembly 324 and extends through anL-shaped bracket arm 255, rotation hub 252, handle receiving hub 256 andhandle mandrel 242. Control wire 326 terminates in a switch 328 throughwhich operation of the monitor 14 can be controlled.

As best shown in FIG. 4, third pivot axis 128 may be longitudinallymoved within pivot pin-receiving aperture 184 by a distance 330 alongradially extending line 194 between an uppermost position 332 and alowermost position 334 by movement of third pivot pin 126 throughrotation of an adjustment screw 336. Radial line 194 forms an angle 338with the diametrical line 190 extending through the first and secondproximal pivot axes 54, 66. Illustratively, angle 338 between radialline 194 and diametrical line 190 is approximately 18.43 degrees. Whencounterbalance arm 40 is moved to its raised position, gas spring 52(coupled through spring links 46 to third pivot axis 128) exerts thestrongest force when the third pivot axis 128 is located in theuppermost position 332. Conversely, when the gas spring 52 is in theraised position, gas spring 52 exerts the weakest force when the thirdpivot axis 128 is in the lowermost position 334. Adjustment screw 336permits third pivot pin 126 to be moved within pivot pin-receivingaperture 184 so that third pivot axis 128 can be located anywherebetween uppermost position 332 and lowermost position 334.

In the illustrated embodiment, a gas spring 52 is used which exhibitsapproximately a 9.5% difference in force between its initial compressedlength and its full-stroke length. The angle 338 formed between theradial line 194 and the diametrical line 190 is fixed to substantiallyreduce, or hopefully eliminate, the difference in force exerted by thegas spring 52 as counterbalance arm is moved between its raised andlowered positions. Thus, angle 338 may be increased or decreased inaccordance with the invention to facilitate other springs exhibitingdifferent “stroke length to force” characteristics. In the illustratedembodiment, angle 338 has a measure of 18.43 degrees.

The measure of the angle 338 was determined through iterations ofexperiments whereby a 3-D computer generated model of counterbalance armwith a known weight attached thereto was generated using knownmeasurements of components of counterbalance arm 40. The model wasgenerated by introducing the arm lengths, displacements between thefirst and second proximal and distal pivot axes, the displacementbetween the top wall of slot 106 and longitudinal axis of secondparallel link 44, the displacement between the pivot pin-receivingaperture and connection pin-receiving aperture 124 of spring link 42,the displacement between the diametrical line 210 and rotation axis 58of the illustrated embodiment of counterbalance arm 40. With thiscomputer model, the location of the third pivot axis with respect to thefirst and second pivot axes could be changed and the modeled arm pivotedto various locations within its pivotal range of motion. In each suchposition, the angle formed by spring link 42 with respect tolongitudinal axis 78 of second parallel link 44 could be measured ascould the location of connecting pin 116 which establishes thedisplacement of the rod of gas spring 52 from its equilibrium position,the length of the moment arm through which spring link 42 and gas springgenerate torque, and the arm angle formed by counterbalance arm. Usingthese various angles and positions of components and a substantiallylinear force to stroke length curve for the gas spring the resultanttorque on the arm for various positions could be calculated and plottedagainst the displacement from diametrical line 190 of third pivot axis128. The arm was positioned in its raised position (38.68 degrees abovethe horizontal or 51.32 degrees from vertical). The third pivot axis 128was moved to an initial location where a substantial equilibrium statewas obtained. Then the counterbalance arm was pivoted to its loweredposition (38.68 degrees below the horizontal or 128.68 degrees from thevertical). The resultant net torque on the arm in the lowered positionwas determined and plotted against the position of third pivot axis 128.Additional torque calculations were taken at various positions betweenthe raised and lowered positions. A new location of third pivot axis 128was chosen and the process repeated. Third pivot axis was moved tovarious locations, but only those locations which produced minimalresultant torques were plotted. A straight line was drawn through theplotted points to establish the appropriate measurement of angle 338. Itis within the teaching of the disclosure for angle 338 to be determinedby other methods so long as the appropriate mechanical disadvantage isprovided. Angle 338 can be determined by experimentation with a physicalmodel or through mathematical modeling.

In generating a mathematical model, or in calculating torques generatedby the weight and spring, the following will be helpful.

While, applicant prefers to select the measurement of angle throughexperimentation and iteration, it is believed that optimal angle can bedetermined generally using mathematical models of counterbalance arm 40.As explained above, first and second distal pivot axes 56 and 70,respectively, are displaced from one another by distance 208 (in theillustrated embodiment equal to 0.75″), shown in FIG. 3, and first andsecond and proximal pivot axes 54 and 66, respectively, are displacedfrom one another by displacement 72 (in the illustrated embodiment equalto 0.75″), shown in FIG. 4, substantially equal to distance 208.Displacement 72 is measure along diametrical line 190 and displacement208 is measured along diametrical line 210. Diametrical line 210 isparallel or substantially parallel to rotation axis 58 which is presumedto be vertical. Distance 211 between diametrical line 210 and rotationaxis 58 is fixed (in the illustrated embodiment 0.781″). Diametricalline 190 is parallel or substantially parallel to rotation axis 34 whichis presumed to be vertical. Distance 191 between diametrical line 190and rotation axis 34 is fixed (in the illustrated embodiment 0.781″).The weight of monitor arm assembly 50 and device 12 is assumed to actvertically downwardly along rotation axis 58. This assumption introducesslight error into the calculation as connections between the distal endof counterbalance arm 40 and components of monitor arm assembly 50 arenot rigid. Assuming that counterbalance arm 40 is a weightlessincompressible moment arm introduces little error to the mathematicalmodeling since monitor arm assembly 50 and device 12 are generally muchmore massive than counterbalance arm 40. Recognizing that some error isintroduced into the mathematical model by these assumptions and otherassumptions to be introduced later, optimal angle can be generallydetermined by assuming that spring force can be linearly modeled usingHooke's Law and that first and second parallel links act as componentsof moment arms to which the law of levers may be applied.

Weight (“W”) of monitor mount assembly 50 and device 12 is assumed to bea determinable constant acting vertically downwardly along rotation axis58 to generate a torque about first proximal pivot axis 54. Thus weightacts on a moment arm having a length 362 (L_(MA1)), as shown in FIG. 8.Arm housing or first parallel link 42, arm weldment or second parallellink 44, proximal arm hub 38 and distal arm hub 48 are configured suchthat pivotal movement of counterbalance arm 40 about first proximalpivot axis 54 is limited. For example, the upper surface of arm housingor first parallel link 42 prohibits rotation of counterbalance arm 40upwardly beyond a point at which upper surface of arm housing 42 wouldcontact the outer surface of proximal hub 38 or bearing housing 36. Inthe illustrated embodiment, hub 38 and first parallel link 42 are formedto include stops (not shown) limiting pivotal movement of counterbalancearm 40 between an upper limit or raised position (wherein the arm angleθ is 38.68 degrees above horizontal or 51.32 degrees from vertical) anda lower limit or lowered position (wherein the arm angle θ is 38.68degrees above horizontal or 128.68 degrees from vertical).

Weight of monitor mount assembly 50 and device 12 thus acts on a momentarm having the length (L_(MA1)) 362 of the distance (L_(1p1)) 55 (in theillustrated embodiment 26″) between first proximal and distal axes 54and 56, respectively, plus the additional distance required to extendthis longitudinal axis to intersect rotation axis 58. Since rotationaxis 58 and the diametrical line 210 through the first and second distalaxes 56 and 70, respectively, is set at a fixed displacement (D) 211(see FIG. 3) and the longitudinal axis forms an angle θ with thevertically oriented line 56 and axis 58, the additional distance isD/sin θ. Thus moment arm has a length 362 (see FIG. 8):L _(MA1) =L _(1p1) +D/sin θ.Since the weight acts vertically downwardly, the component of the weightperpendicular to the moment arm is W*sin θ. Thus the torque generated bythe weight can be expressed:τ=−W*sin θ*(L _(1p1) +D/sin θ)=−(W*L _(1p1)*sin θ+W*D)

Referring again to FIGS. 3 and 5, to maintain equilibrium of thecounterbalance arm 40, the force exerted by the gas spring 52 mustgenerate a torque having the same magnitude in the opposite direction.The force (F) exerted by the gas spring 52 acts on connection pin 116extending through slot 106 and connection pin-receiving aperture 124 indistal end 122 of spring link 46. Spring link 46 and connection pin 116and top wall 108 of slot 106 act to reduce the spring force to acomponent along the longitudinal axis of the spring link 46 and a forceperpendicular to the longitudinal axis 78 of second parallel link 44.Counterbalance arm 40 is formed to maintain longitudinal axis 78 ofsecond parallel link 44 parallel to longitudinal axis 76 of firstparallel link 42. Proximal hub 38 is formed to fix first and secondproximal pivot axes 54, 66 so that second pivot axis 66 is alwaysvertically above first pivot axis 54 and displaced from first pivot axis54 by distance 72. Distal hub 48 is formed to fix first and seconddistal pivot axes 56, 70 so that second pivot axis 70 is alwaysvertically above first pivot axis 56 and displaced from first pivot axis56 by distance 208. In the illustrated embodiment, top wall 108 of slot106 is displaced from longitudinal axis 78 of second parallel link 42 bya displacement of 0.125″. This displacement is minimal compared to otherrelevant lengths, distances and displacements and may be ignored inmodeling counterbalance arm 40 with little error introduction. The forceperpendicular to the longitudinal axis 78 of second parallel link 44generates a torque in the opposite direction from the torque generatedby the weight W. Because of the parallel construction, the forceperpendicular to the longitudinal axis 78 of second parallel link 44 maybe treated for modeling purposes as a force perpendicular to thelongitudinal axis 76 of second parallel link 42 and as generating atorque in the opposite direction from the torque generated by the weightW. However in order to satisfy this simplification, the focus of angle332 must be displaced downwardly by the distance 72 between the firstand second proximal pivot pins

Those skilled in the art will recognize that when counterbalance arm 40is in its lowered position, forming an angle of 128.68 degrees to thevertical, the weight of monitor arm assembly 50 and device 12 generatesa torque about first proximal pivot axis 54 identical in magnitude tothe torque generated when the counterbalance arm is in the raisedposition, forming an angle of 128.68 degrees to the vertical, becausesin(51.32) is equal to sin(128.68). However, the angle formed betweenspring link 46 and second parallel link 44 decreases as counterbalancearm 40 rotates between its raised and lowered positions. The decrease ofthe angle formed between spring link 46 and second parallel link 44causes the component of force perpendicular to second parallel link 44to be decreased. Similarly, as counterbalance arm 40 pivots from raisedposition toward lowered position, the length of the moment arm at whichthe component of spring force perpendicular to second link actsincreases, thereby reducing the torque generated proportionally.

Those skilled in the art will recognize that the device disclosed hereintakes advantage of offsetting the third pivot axis 128 downwardly fromthe first and second proximal pivot axes 54 and 66, respectively andinwardly between the line 190 intersecting the first and second proximalpivot axes 54 and 66 and the line 210 intersecting the first and seconddistal pivot axes 56 and 70 respectively to provide a mechanicaldisadvantage to the combination of spring link 46 and spring 136 throughthe range of pivotal motion of counterbalance arm 40. Preferably themechanical disadvantage compensates for the increased force exerted byspring 136 resulting from its further displacement from equilibriumduring pivotal movement of counterbalance arm 40 between its upper limitof pivotal motion and its lower limit of pivotal motion. To the extentthat the mechanical disadvantage does not completely compensate for theincreased force exerted by spring 136, frictional forces inherent in thecounterbalance arm 40 permit the counterbalance arm to maintain anequilibrium state at each orientation throughout its range of pivotalmotion.

Referring to FIG. 6, counterbalance arm 40 is shown in the raisedposition with adjustment screw 336 rotated so that the third proximalpivot axis 128 is in its uppermost position causing the spring 136 toexert its strongest initial force. Whenever counterbalance arm 40 is inits raised position, i.e. at its upper limit regarding its pivotalmovement about first proximal pivot axis 54, connection pin 116extending through connection pin-receiving hole 124 in distal end 122 ofspring links 46 is at a minimal displacement 350 from proximal end wall114 of slot 106. The minimal displacement 350 of connection pin 116 fromproximal end wall 114 differs depending on the location at which thethird proximal pivot axis 128 is set. When the third proximal pivot axis128 is set at its uppermost position, as shown, for example, in FIG. 6,the minimal displacement 350 of connection pin 116 from proximal endwall 114 of slot 106 is at a maximum value 352. Distal clevis 104 of rod134 pivots about pivot pin 102 which is in a fixed position relative toarm weldment 44 and proximal clevis 130 of cylinder housing 132 ismovable longitudinally with respect to arm weldment 44. Thus, when thirdproximal pivot axis 128 is in its uppermost position and the minimaldisplacement 350 of connection pin 116 from proximal end wall 114 is atits maximum value 352, the cylinder head within gas spring 52 has beenurged by rod 134 to begin compressing gas in the cylinder housing 132 bya maximum initial value causing the force exerted by the gas spring 52to be at a maximum initial value.

Referring to FIG. 7, counterbalance arm 40 is shown in the raisedposition with the adjustment screw 336 rotated so that the thirdproximal pivot axis 128 is in its lowermost position causing the spring136 to exert its weakest initial force. When the third proximal pivotaxis 128 is set at its lowermost position, as shown, for example, inFIG. 7, the minimal displacement 350 of connection pin 116 from proximalend wall 114 of slot 106 is at minimum value 354. Distal clevis 104 ofrod 134 pivots about pivot pin 102 which is in a fixed position relativeto arm weldment 44 and proximal clevis 130 of cylinder housing 132 ismovable longitudinally with respect to arm weldment 44. Thus, when thirdproximal pivot axis 128 is in its lowermost position and the minimaldisplacement 350 of connection pin 116 from proximal end wall 114 is atits minimum value 354, the cylinder head within gas spring 52 has beenurged by rod 134 to begin compressing gas in the cylinder housing 132 bya minimum initial value causing the force exerted by the gas spring 52to be at a minimum initial value.

As shown for example, in FIG. 8, when counterbalance arm 40 is rotateddownwardly about first proximal pivot axis 54, connection pin 116extending through connection pin-slides receiving hole 124 in distal end122 of spring links 46 and proximal clevis 130 of gas spring 52 slideslongitudinally within slot 106 to increase a displacement 356 ofconnection pin 116 from proximal end wall 114 of slot 106. As thedisplacement 356 of connection pin 116 from proximal end wall 114increases, rod 134 urges cylinder head of gas spring 52 further intocylinder housing 132 further compressing the gas in the cylinder andincreasing the force exerted by the gas spring 52.

As shown, for example, in FIG. 9, when counterbalance arm 40 is rotateddownwardly about first proximal pivot axis 128 to its lowered position,connection pin 116 extending through connection pin-receiving hole 124in distal end 122 of spring links 46 and proximal clevis 130 of gasspring 52 slides longitudinally within slot 106 so that the displacement356 of connection pin from proximal end wall 114 of slot 106 reaches amaximum value 358. As the displacement 356 of connection pin 116 fromproximal end wall 114 increases, rod 134 urges cylinder head of gasspring 52 further into cylinder housing 132 further compressing the gasin the cylinder and increasing the force exerted by the gas spring 52.Thus, when the displacement 356 reaches a maximum value 358, the forceexerted longitudinally along arm weldment 44 by spring 136 reaches amaximum value for a set location of the third proximal pivot pin 126.

As shown in FIGS. 7–9, because first parallel link 42, second parallellink 44 and the fixed displacement 72 of first and second proximal pivotaxes 54, 66, respectively, and first and second distal pivot axes 56,70, respectively, create a parallelogram linkage, diametrical lines 190,210 remain parallel throughout the pivotal range of counterbalance arm40 about first proximal pivot axis 54. This permits monitor 14 mountedto monitor mount assembly 50 to remain properly oriented regardless ofits vertical displacement from ceiling 20 of surgical suite.

As mentioned in the background, spring links 46, connection pin 116,slot 106 and gas spring 52 cooperate to urge connection pin 116 againstupper wall 108 of slot 106 so that an upwardly directed verticalcomponent of force is exerted at the location of connection pin 116 onsecond link 44. First parallel link 42 can be viewed as a lever with afulcrum at the first proximal pivot axis 54. Thus, to facilitatevertical positioning of a device 12 connected to monitor mount assembly50, it is desirable that the vertical component of force exerted by theconnection pin 116 multiplied by the length 360 of the moment arm fromthe first proximal pivot axis 54 to the location of the connection pin116 be equal to the vertical component of the force exerted by theweight of the monitor mount and device and the length 362 of the momentarm between the first proximal pivot axis 54 and rotation axis 58.Illustratively, angle 338 between radial line 194 and diametrical line190 is determined based on the length of the spring links 46, the length362 of the moment arm from first proximal pivot axis 54 to rotation axis58, the displacement 72 between the first and second parallel links, andthe force to stroke length characteristics of spring 136, so that theproper mechanical disadvantage is provided to cause the torque exertedby the weight of the monitor mount assembly 50 and device 12 to thecounterbalance arm 40 to be substantially equal to, but opposite indirection from, the torque applied by the connection pin 116 through theupper wall 108 of the slot 106 to counterbalance arm 40.

Although the foregoing embodiment has been described, one skilled in theart can easily ascertain the essential characteristics of the apparatus,and various changes and modifications may be made to adapt the varioususes and characteristics without departing from the spirit and scope ofthis application, as described by the claims which follow.

1. A counterbalance arm assembly comprising: a proximal hub formed toinclude a first proximal pivot axis extending therethrough, a secondproximal pivot axis extending therethrough, and a third proximal pivotaxis extending therethrough, said first, second and third proximal pivotaxes being substantially mutually parallel, wherein the displacementbetween the first and third proximal pivot axes is adjustable, a distalhub having a first distal pivot axis extending therethrough and a seconddistal pivot axis extending therethrough, a first parallel link coupledat its proximal end for pivotal movement of the first parallel linkrelative to the proximal hub about the first proximal pivot axis andcoupled at its distal end for pivotal movement of the first parallellink relative to the distal hub about the first distal pivot axis, asecond parallel link coupled at its proximal end for pivotal movement ofthe second parallel link relative to the proximal hub about the secondproximal pivot axis and coupled at its distal end for pivotal movementof the second parallel link relative to the distal hub about the seconddistal pivot axis, a spring coupled at its distal end to the secondparallel link and at its proximal end to the second parallel link forlongitudinal movement of the proximal end of the spring relative to thesecond link, a spring link coupled at its proximal end to the proximalhub for pivotal movement of the spring link about the third proximalaxis, coupled at its distal end to the second parallel link forlongitudinal movement of the distal end of the spring link relative tothe second parallel link, and coupled at its distal end to the proximalend of the spring, wherein the proximal hub is formed to include anaperture extending radially from the first pivot axis and furthercomprising a pivot pin defining the third proximal pivot axis, said pinbeing slidably received in the aperture for movement of the pin radiallywith respect to the first proximal pivot axis, wherein a lineintersecting the first and third proximal pivot axes is not colinearwith and forms an angle with a line intersecting the first and secondproximal pivot axes, wherein the first parallel link includes alongitudinal axis intersecting the first proximal and distal pivot axes,the second parallel link includes a longitudinal axis substantiallyparallel to the longitudinal axis of the first parallel link andintersecting the second proximal and distal pivot axes, the longitudinalaxes of the first and second parallel links remain substantiallyparallel during movement of a device coupled thereto between a firstelevation and a second elevation and the first longitudinal axis isbetween the second and third pivot axes.
 2. The counterbalance armassembly of claim 1 wherein a line intersecting the first and seconddistal pivot axes remains parallel to the line intersecting the firstand second proximal pivot axes during movement of the device between thefirst elevation and the second elevation and the third proximal pivotaxis is disposed between the line intersecting the first and seconddistal pivot axes and the line intersecting the first and secondproximal pivot axes.
 3. The counterbalance arm assembly of claim 2wherein the first parallel link includes a housing having a cavityextending therethrough and a portion of the second parallel link isreceived in the cavity.
 4. The counterbalance arm of claim 3 furthercomprising caps and plugs and wherein the first parallel link, caps,plugs, proximal hub and distal hub cooperate to form an enclosure aroundthe second parallel link.
 5. A surgical theater system for attachment toa structural component in a ceiling of a surgical suite, the surgicaltheater system comprising: a hub configured to be mounted to thestructural component and configured to rotate about a first verticalaxis extending through the hub, a boom having a proximal end and avertically-downwardly extending portion horizontally displaced from theproximal end terminating in a distal end, the boom being mounted at itsproximal end to the hub, the vertically-downwardly extending portionhaving a second vertical axis extending therethrough, a proximal hubformed to include a first proximal pivot axis extending therethrough, asecond proximal pivot axis extending therethrough, and a third proximalpivot axis extending therethrough, an aperture extending radially fromthe first pivot axis, and including a pivot pin defining the thirdproximal pivot axis slidably received in the aperture for movement ofthe pin radially with respect to the first proximal pivot axis wherebythe displacement between the first and third proximal pivot axes isadjustable, the proximal hub being mounted to the vertically-downwardlyextending portion of the boom for rotational movement of the hub aboutthe second vertical axis, a distal hub having a first distal pivot axisextending therethrough and a second distal pivot axis extendingtherethrough, a first parallel link coupled at its proximal end forpivotal movement of the first parallel link relative to the proximal hubabout the first proximal pivot axis and coupled at its distal end forpivotal movement of the first parallel link relative to the distal hubabout the first distal pivot axis, a second parallel link coupled at itsproximal end for pivotal movement of the second parallel link relativeto the proximal hub about the second proximal pivot axis and coupled atits distal end for pivotal movement of the second parallel link relativeto the distal hub about the second distal pivot axis, a spring coupledat its distal end to the second parallel link and at its proximal end tothe second parallel link for longitudinal movement of the proximal endof the spring relative to the second link, a spring link coupled at itsproximal end to the proximal hub for pivotal movement of the spring linkabout the third proximal axis, coupled at its distal end to the secondparallel link for longitudinal movement of the distal end of the springlink relative to the second parallel link, and coupled at its distal endto the proximal end of the spring.
 6. The surgical theater system ofclaim 5 wherein a line intersecting the first and third proximal pivotaxes is not colinear with and forms an angle with a line intersectingthe first and second proximal pivot axes.
 7. The surgical theater systemof claim 6 wherein the first parallel link includes a longitudinal axisintersecting the first proximal and distal pivot axes, the secondparallel link includes a longitudinal axis intersecting the secondproximal and distal pivot axes, the longitudinal axes of the first andsecond parallel links remain parallel during movement of a devicecoupled thereto between a first elevation and a second elevation and thefirst longitudinal axis is between the second and third pivot axes and aline intersecting the first and second distal pivot axes remainsparallel to the line intersecting the first and second proximal pivotaxes during movement of the device between the first elevation and thesecond elevation and the third proximal pivot axis is disposed betweenthe line intersecting the first and second distal pivot axes and theline intersecting the first and second proximal pivot axes.
 8. Acounterbalance arm assembly comprising: a proximal hub formed to includea first proximal pivot axis extending therethrough, a second proximalpivot axis extending therethrough, and a third proximal pivot axisextending therethrough, wherein a first line intersects the first andsecond proximal pivot axes and the displacement between the first andthird proximal pivot axes is adjustable, a distal hub having a firstdistal pivot axis extending therethrough and a second distal pivot axisextending therethrough, wherein a second line intersects the first andsecond distal pivot axes and is substantially parallel to the firstline, a first parallel link coupled at its proximal end for pivotalmovement of the first parallel link relative to the proximal hub aboutthe first proximal pivot axis and coupled at its distal end for pivotalmovement of the first parallel link relative to the distal hub about thefirst distal pivot axis, the first parallel link having a firstlongitudinal axis intersecting the first proximal and distal pivot axesa second parallel link coupled at its proximal end for pivotal movementof the second parallel link relative to the proximal hub about thesecond proximal pivot axis and coupled at its distal end for pivotalmovement of the second parallel link relative to the distal hub aboutthe second distal pivot axis, the second parallel link having a secondlongitudinal axis intersecting the second proximal and distal pivot axesand is substantially parallel to the first longitudinal axis, a springcoupled at its distal end to the second parallel link and at itsproximal end to the second parallel link for longitudinal movement ofthe proximal end of the spring relative to the second link, a springlink coupled at its proximal end to the proximal hub for pivotalmovement of the spring link about the third proximal axis, coupled atits distal end to the second parallel link for longitudinal movement ofthe distal end of the spring link relative to the second parallel link,and coupled at its distal end to the proximal end of the spring, andwherein the third proximal pivot axis has a range of motion over whichthe third proximal pivot axis is always positioned so that the firstlongitudinal axis is between the second longitudinal axis and the thirdproximal pivot axis and so that the third proximal pivot axis is betweenthe first line and the second line, whereby a mechanical disadvantage isprovided to the coupled spring link and spring substantiallycompensating for increased spring forces resulting from furtherdisplacement from the spring's equilibrium length during pivotalmovement of the counterbalance arm between a raised and a loweredposition, and wherein the second distal link includes alongitudinally-extending slot and a connecting pin, said connecting pinextending laterally through the slot to couple the distal end of thespring link and the proximal end of the spring to the second parallellink.
 9. The counterbalance arm of claim 8 wherein the first parallellink includes a housing having a cavity extending therethrough and aportion of the second parallel link is received in the cavity.
 10. Thecounterbalance arm of claim 9 wherein the first parallel link, proximalhub and distal hub cooperate to enclose the second parallel link.